Since the beginning of the electronic era, the primary building blocks in electronics and microelectronics have been field-effect transistors (FETs) based on inorganic electrodes, insulators, and semiconductors. These materials have proven to be reliable and highly efficient, providing performance that improves continually according to Moore's law. More recently, organic materials have been developed as both active and passive materials in electronic circuitry. Instead of competing with conventional silicon technologies, organic FETs (OFETs) based on molecular and polymeric materials are desired in niche applications, for example, in low-end radio-frequency technologies, sensors, and light emission, as well as in integrated optoelectronic devices such as pixel drives and switching elements in displays. These systems have been widely pursued for the advantages they offer, which include processability via vapor/solution-phase fabrication, good compatibility with different substrates (e.g., flexible plastics), and opportunities for structural tailoring. This trend is further driven by the continued demand for low-cost, large-area, flexible, and lightweight devices, and the possibility to process these materials at much lower substrate temperatures compared to inorganic semiconductors.
The simplest and most common OFET device configuration is that of an organic thin-film transistor (OTFT), in which a thin film of the organic semiconductor is deposited on top of a dielectric with an underlying gate (G) electrode. Charge-injecting drain-source (D-S) electrodes providing the contacts are defined either on top of the organic film (top-configuration) or on the surface of the FET dielectric prior to the deposition of the semiconductor (bottom-configuration). The current between the S and D electrodes is low when no voltage (Vg) is applied between the G and D electrodes, and the device is in the so called “off” state. When Vg is applied, charges can be induced in the semiconductor at the interface with the dielectric layer. As a result, current (Id) flows in the channel between the S and D electrodes when a source-drain bias (Vd) is applied, thus providing the “on” state of a transistor. Key parameters in characterizing FET performance are the field-effect mobility (μ), which quantifies the average charge carrier drift velocity per unit electric field, and the current on/off ratio (Ion:Ioff), which is the D-S current ratio between the “on” and “off” states. For a high-performance OFET, the field-effect mobility and on/off ratio should both be as high as possible, for example, having at least μ˜0.1-1 cm2V−1 s− and Ion/Ioff˜106.
Most OFETs operate in p-type accumulation mode, meaning that the semiconductor acts as a hole-transporting material. However, high-performance electron-transporting (n-type) materials are needed as well. For most practical applications, the mobility of the field-induced charges should be greater than about 0.01-1 cm2/Vs. To achieve high performance, the organic semiconductors should satisfy stringent criteria relating to both the injection and current-carrying capacity; in particular: (i) the HOMO/LUMO energies of the material should be appropriate for hole/electron injection at practical voltages; (ii) the crystal structure of the material should provide sufficient overlap of the frontier orbitals (e.g., π-stacking and edge-to-face contacts) to allow charges to migrate among neighboring molecules; (iii) the compound should be very pure as impurities can hinder the mobility of charge carriers; (iv) the conjugated core of the material should be preferentially oriented to allow charge transport in the plane of the TFT substrate (the most efficient charge transport occurs along the direction of intermolecular π-π stacking); and (v) the domains of the crystalline semiconductor should uniformly cover the area between the source and drain contacts, hence the film should have a single crystal-like morphology.
Among the organic semiconductors used in OFETs, oligothiophenes, polythiophenes, acenes, rylenes, and phthalocyanenes are the most investigated. For instance, the first report on a polyheterocycle-based PET was on polythiophene. In addition, poly(3-hexyl)thiophene and α,ω-dialkyloligothiophenes were the first high-mobility polymer and small molecules, respectively. Over the years, chemical modifications of these compounds, including variations in ring-to-ring connectivity and substitution pattern, have resulted in a considerable number of electro-active materials. However, with very few exceptions, all of these materials are p-type semiconductors. One exception is an alternating copolymer of perylene diimide and dithienothiophene units which was reported to have electron mobilities as high as 1.3×10−2 cm2V−1 s−1 and an on/off current ratio of >104 in vacuum. See, e.g., Zhan, X. et al., J. Amer. Chem. Soc. 129:7246-7247 (2007).
In addition to the scarcity of n-type semiconductor materials, OFET electron transport is frequently depressed, or even completely quenched, when the devices are operated in the presence of ambient species (e.g. oxygen, water, and carbon dioxide). This sensitivity to ambient conditions severely hinders the ability to operate these devices without proper encapsulation.
Another important class of organic semiconductor-based devices where electron-transporting (n-type) materials are needed is bulk heterojunction photovoltaic (or solar cell). In these devices, the combination of an electron donor semiconductor (e.g., poly(3-hexylthiophene (P3HT)) and an electron acceptor semiconductor (e.g., methanofullerene[6,6]-phenyl-butyric acid methyl ester (PCBM)) work together to split the exciton (hole-electron pair formed upon light absorption) and generate power. It is desirable that both the electron-donor semiconductor and the electron-acceptor semiconductor possess a broad optical absorption so that they are able to absorb as much light from the solar spectrum as possible. For example, a drawback of PCBM is that it does not absorb light in the visible/near IR part of the spectrum.
The synthesis of a large number of electron-acceptor (π-acceptor) carbonyl-functionalized oligothiophenes was recently described and compared to the molecular/solid-state properties of the corresponding alkyl-substituted and parent unsubstituted oligothiophenes. Each of these substituted oligothiophenes exhibits high chemical/thermal stability, similar packing characteristics, strong π-π intermolecular interactions, and low LUMO energies. Furthermore, carbonyl functionalization of the oligothiophene core was found to have a significant impact on the electronic, film growth, and semiconducting properties of the resulting films, and TFT devices with such systems as the active layer were demonstrated to operate in the n-type accumulation mode, indicating facile electron injection into the semiconductor material. See, e.g., U.S. Pat. Nos. 6,585,914, 6,608,323, 6,991,749, and U.S. Patent Application Publication Nos. 2006/0124909 and 2006/0186401, the entire disclosure of each of which is incorporated by reference herein.
A separate class of electron-acceptor-functionalized (e.g., cyano-substituted) rylene imide-based semiconductors was shown to exhibit excellent stable operation in air. Data from relevant studies suggest that electron transport in these molecules is possible in air if the electron affinity (EA) or the first reduction potential (the equivalent solution state parameter) of the molecule is sufficiently increased or sufficiently negative, respectively. Although it is difficult to pinpoint the exact EA required for the onset of such stability, it appears that for rylene-containing molecules it occurs in the range of about −3.9 eV to about −4.4 eV versus vacuum. See, e.g., U.S. Patent Application Publication No. 2005/0176970, the entire disclosure of which is incorporated by reference herein.
In addition to the various deficiencies described above, molecular semiconductors generally have limited processability. High-performance p-channel polymers with hole mobilities of about 0.1 cm2V−1 s−1 have been reported, but n-channel polymers for OTFTs to date either suffer from poor processability and/or negligible electron mobilities under ambient conditions.
Accordingly, new classes of polymers having semiconducting activity are desired in the art, especially those with n-type semiconducting activity, are stable at ambient conditions, and/or can be processed in solution-phase (e.g., via printing, casting, spraying, or spin-coating).